The invention relates to a displacement device with a first part and a second part which are displaceable relative to one another in at least one direction (x), said first part comprising a carrier which extends substantially parallel to the displacement direction and on which a system of magnets is fastened in accordance with a pattern whereby magnets of a first kind (N) with a magnetization direction perpendicular to the carrier and directed towards the second part and magnets of a second kind (Z) with a magnetization direction perpendicular to the carrier and directed away from the second part are arranged so as to alternate with one another, which system of magnets has a magnetic flux density which repeats itself periodically in accordance with B(x)=B(x+2xcfx84), in which xcfx84 is the magnetic pole pitch, while the second part is provided with an electric coil system with at least one coil block unit comprising k coil blocks, with kxe2x89xa72, which coil blocks have current conductors which are situated in a magnetic field of the system of magnets, all current conductors belonging to one coil block unit being oriented in the same direction and being fed by an n-phase system, with nxe2x89xa72, while furthermore the magnets of the magnetic system are arranged such that, viewed in a direction perpendicular to the current conductors, the flux of the magnetic system varies substantially sinusoidally in accordance with   Φ  =      sin    ⁢                            π          ⁡                      (                          x              +                              x                0                                      )                                    2          ⁢          τ                    .      
Such a displacement device is known from U.S. Pat. No. 4,654,571. This displacement device is used in a lithographic device (wafer stepper) for the manufacture of semiconductors. Said second part with the electric coil system is the part moving in the X-Y plane in this case. A problem in such displacement devices is that, during the displacement of the moving part, this part is subject to changing forces in the Z-direction, so that a continuously varying torque will be exerted on the moving part about an axis lying in the X-Y plane. This causes an additional rocking movement which adversely affects the displacement accuracy. It will be explained in more detail with reference to FIG. 8 what gives rise to this rocking movement. A situation is depicted in a linear arrangement for the sake of simplicity, i.e. a system of magnets 1 with N- and Z magnets alternating in the X-direction, while a coil block 2 with six current conductors 3 is present in the magnetic field, said current conductors being supplied by a 3-phase system. Such a system of magnets complies with the conditions mentioned in the first paragraph. The current conductors extend in the Y-direction, i.e. transversely to the direction of movement. The current conductors 3 are subject to a magnetic flux density which varies its direction and magnitude in dependence on their position. The vectors X, Y, and Z of the flux density have a sinusoidal gradient. When a current is passed through the current conductors, the flux density Bz exerts a Lorentz force Fx on the current conductors and accordingly on the coil block 2 in a direction perpendicular to the current conductors and the flux density, i.e. in the X-direction. It is possible to achieve a constant force Fx through a correct distribution of currents in the conductors (commutation), and thus to obtain a controlled movement of the coil block. This current distribution, however, also leads to local Fz forces whose sum is indeed zero in the end, but which do exert a torque My on the current conductors 2. These local Fz forces are the result of the occurring local values of the flux density Bx and the local values of the currents through the conductors, for which it is true that       B    x    =                              B          ⋒                x            ⁢      sin      ⁢                        π          ⁢                      xe2x80x83                    ⁢          x                          2          ⁢          τ                    ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢              I                  y          ,          ph                      =                            I          ⋒                          y          ,          ph                    ⁢                        sin          ⁡                      (                                          π                                  2                  ⁢                                      xe2x80x83                                    ⁢                  τ                                            +                              p                ⁢                                  xe2x80x83                                ⁢                h                ⁢                                  xe2x80x83                                ⁢                                                      2                    ⁢                    π                                    n                                            +              ϕ                        )                          .            
Here ph is the number of the relevant phase, n is the number of phases with which the current conductors are supplied (in the example of FIG. 5: n=3), and xcfx89 is the phase angle (0-2xcfx80). The values of the local forces Fz with respect to the mass center M of the coil block 2 change during the movement of the coil block in the X-direction. The magnitude of the torque thus changes continually and has a repetitive character. It is this torque which causes the undesired rocking effect on the coil block. It will be obvious that this adverse phenomenon will also arise in both directions X and Y when the system of magnets extends along the X-Y plane and the coil block can move in the X- and Y-direction, as is the case in U.S. Pat. No. 4,654,571.
It is an object of the invention to improve the displacement device as described in the opening paragraph such that said rocking effect during the displacement of the moving part in the X- and/or Y-direction does not occur any more or is at least as small as possible.
The displacement device is for this purpose characterized in that it is true for all coil blocks belonging to one coil block unit that the distance between mass centers of consecutive coil blocks, viewed in a direction perpendicular to the conductors, is substantially equal to             τ      k        +          p      xc3x97      τ        ,
with p=2, 3, . . .
The provision of equally oriented coil blocks with interspacings as indicated above in a coil block unit gives rise to torques My on each coil block which are approximately of the same value, but oppositely directed, so that these torques cancel each other out as much as possible, and the rocking effect is minimized.
A preferred embodiment of the displacement device is characterized in that the parts are displaceable relative to one another in an X-direction, the magnets of the different kinds extending alternately and substantially exclusively in the X-direction, while the magnetic pole pitch xcfx84 is defined as the distance between center points of two mutually adjoining magnets of different kinds (N and Z), and the current conductors of the coil blocks are oriented substantially perpendicularly to the X-direction. The moving part is displaced exclusively in the X-direction in such an arrangement. This displacement device is also referred to as a linear motor.
If the moving part is to be capable of displacements both in the X- and in the Y-direction, the device is characterized in that the parts are displaceable relative to one another in an X-direction and/or in a Y-direction perpendicular to the X-direction, the magnets of the system of magnets of the first part being arranged in accordance with a pattern of rows and columns perpendicular thereto and enclosing an angle of approximately 45xc2x0 with the X-direction, while magnets of a first kind (N) with a magnetization direction perpendicular to the carrier and directed towards the second part and magnets of a second kind (Z) with a magnetization direction perpendicular to the carrier and directed away from the second part are arranged in each row and in each column alternately, the magnetic pole pitch xcfx84 being defined as the distance between two mutually adjoining diagonal lines on which center points of magnets of the same kind (N or Z) are situated, and the coil system of the second part comprises at least two coil block units, current conductors of at least one of said coil block units being oriented in the X-direction and of the other coil block units in the Y-direction. Such a displacement device is also called a planar motor.
A further embodiment of the displacement device is characterized in that a magnet of a third kind (H) with a magnetization direction parallel to the X-Y plane and directed towards the magnet of the first kind (N) is present between each magnet of the first kind (N) and of the second kind (Z). The magnets are placed in a so-called Halbach arrangement here. Such an arrangement leads to an intensification of the magnetic field, so that the forces in the direction of movement are greater. A compensation of the My torques takes place also in this arrangement.